picnic vs field day

picnic and field day both are nouns.

Word	Noun	Adjective	Verb	Adverb
picnic	Yes	No	Yes	No
field day	Yes	No	No	No

As nouns, picnic and field day are synonyms defined as:

Other synonyms of picnic include outing.

picnic (noun)	field day (noun)
any undertaking that is easy to do	a day for outdoor athletic competition
any informal meal eaten outside or on an excursion	a day devoted to an outdoor social gathering
a day devoted to an outdoor social gathering	(military) a day for military exercises and display
	a time of unusual pleasure and success

picnic (verb)	field day (verb)
eat alfresco, in the open air